1. Field of the Invention
The present invention relates to an X-ray inspection apparatus, and specifically, to an X-ray microscopic inspection apparatus capable of providing better resolution than 0.1 μm over a broad range of an accelerating voltage by using an electron source for emitting a high intensity electron flow and a lens system for focusing electrons on the X-ray target.
2. Description of the Related Art
As an inspection apparatus utilizing an X-ray, various kinds of industrial inspection apparatuses such as an X-ray microscope, a foreign body inspection apparatus, a fluorescent X-ray analyzing apparatus, and medical X-ray apparatuses such as an X-ray diagnostic apparatus are known. FIG. 1 shows a construction example of a conventional X-ray inspection apparatus. The X-ray inspection apparatus in this example is designed so as to obtain a micro X-ray point source 23a by accelerating electrons Re from an electron source 21b by applying a high voltage between a grid 21a and an anode 21c using a thermionic emission cathode 21b as the electron source, and then focusing the electrons Re on a target 23 formed by a thin plate of high-melting point metal such as tungsten by electron lenses 22. Subsequently, the inside of a sample (object to be inspected) 10 is projected in magnifying mode by using the point-form X-ray Rx generated from the X-ray targets 23a and the microstructure inside of the sample is subjected to non-destructive perspective inspection.
In such X-ray inspection apparatus, the electron beam Re impinging on the target 23 is converted into the X-ray Rx thereon, however, its conversion efficiency is as extremely low as equal to or less than 1%, and most of the energy of the electron beam Re is converted into heat on the target 23. By the way, since an X-ray has no electric charge, it can not be bent freely as an electron by using an electron lens. On this account, in order to obtain high magnifying power, it is necessary to bring the sample 10 as near to the X-ray source 23a as possible, to capture the X-ray Rx that is transmitted through the sample 10 and spreads out radially with a two-dimensional detector (X-ray detector) 24 disposed at a distance as far as possible, and to make it into an image (there are various kinds of X-ray detectors 24, and an X-ray is converted into light and subjected to amplification and imaging). Only in theory, the magnifying power is infinitely increased as the distance between the sample 10 and the X-ray detector 24 is taken larger, however, actually, since the X-ray amount per unit area is reduced in inverse proportion to the square of the distance, the upper limit of the magnifying power is determined by the balance between the sensitivity of the X-ray detector 24 and the X-ray amount or X-ray density on the X-ray detector of the magnified image.
On the other hand, the resolving power of the X-ray image transmitted through the sample 10 is more improved by making the X-ray source size (focal point size) smaller because the blurring amount is reduced. In the case where the same electron source 21b is used, the X-ray source size can be made smaller by focusing the electron into a small spot by the electron lens 22, however, since the electron beam amount included therein is reduced in reverse proportion to the square of the spot diameter and the X-ray amount is also reduced in response thereto, the final resolving power is determined by the balance between the electron spot diameter in which enough X-ray amount is produced and the sensitivity of the above described X-ray detector 24, and has a certain limit. In the conventional X-ray microscopic inspection apparatus that the applicant has developed and commercialized, a two-stage reduction system using lenses having as small spherical aberration and chromatic aberration as possible for the focusing lens system and a LaB6 (lanthanum hexaboride) cathode having an advantageous character as a thermionic source are adopted, and further, an image intensifier with high sensitivity is used, and thereby the resolving power becomes less than 1 μm and achieves about 0.4 μm. This is the highest value on a global basis as a practical X-ray inspection apparatus at present (the degree of 0.1 μm is the highest value if the exposure time is neglected), and the value may be assumed as the technical limit under the present circumstances. Therefore, the resolving power better than 0.1 μm expected in the invention can not be implemented by the conventional technology (see the following description of the non-patent documents).
Hereinafter, the conventional technology concerning the resolving power of the X-ray inspection apparatus will be described.
The technology concerning the resolving power is disclosed in Non-patent Document 1, Nixon, “High-resolution X-ray projection microscopy”, 1960, A232: pp. 475-485, Non-patent Document 2, Keiji Yada & Hisashi Ishikawa, “Projection X-ray Shadow Microscopy using SEM”, Bulletin of the Research Institute for Scientific Measurements, Tohoku University, 1980, Vol. 29, No. 1, pp. 25–42, Non-patent Document 3, Keiji Yada & Kunio Shinohara, “Development of Soft X-ray Microscopy”, 1980, Biophysics, Vol. 33, No. 4, pp. 8–16, Non-patent Document 4, Keiji Yada & Shoichi Takahashi, “High-Resolution Projection X-ray Microscopy”, 1994, Chap. 8, pp. 133–150, and Non-patent Document 5, Keiji Yada & Kunio Shinohara, “Development of Projection X-Ray Microscopy and Its Biological Applications” 1996, Bulletin of Aomori Public College, Vol. 1, pp. 2–13, for example. In Non-patent Document 1, there described that, regarding X-ray Shadow Microscopy, the limit of its resolving power has been 0.5 μm conventionally, however, the resolving power of 0.1 μm is achieved by using a high brightness electron emitter and a very thin metal film (0.1 μm in thickness) as the target at this time. In addition, there also described that the exposure time for obtaining a sheet of image is five minutes, and after Non-patent Document 1 is disclosed, studies for shortening the exposure time have been actively performed. Further, Non-patent Document 2 is a research report (bulletin of the research institute for scientific measurements, Tohoku University) on the projection X-ray shadow microscopy utilizing an irradiation system of an electron microscope, and there described that the resolving power of 0.1 μm is achieved. Additionally, theoretical analyses are performed regarding respective factors that affect the resolving power, and there derived the conclusion that the spot size of the X-ray source exerts the greatest effects on the resolving power. Furthermore, there described that, by converting a SEM (scanning electron microscope) to an X-ray microscope, scanning of the electron beam with a deflection coil is utilized for focusing.
Moreover, Non-patent Document 3 is for explaining the trend in the X-ray microscopy to the present, and there explained that the soft X-ray microscope of a relatively short wavelength (0.1 to 10 nm) by specifically referring to the observation of biological samples. The contents of Non-patent Document 4 are substantially the same as those of Non-patent Document 2, however, there shown a densitometry profile of an X-ray image having the resolving power better than 0.1 μm (on 146 page in the main body). Non-patent Document 5 is for explaining the X-ray microscope in an easily understandable way, and there described that the image quality becomes better by changing the target in relation to the sample that is difficult to provide contrast as is the case with Non-patent Documents 2, 3, and 4.
In order to manufacture an X-ray inspection apparatus having high power resolution never before possible, an electron source with higher brightness (greater current amount per unit area/unit solid angle) and greater emission current amount becomes required. Additionally, an electron lens system for assuring a great electron probe current amount as possible becomes also required. Further, devices for increasing the heat release effect of the target are required so that the target may not melt or evaporate even if the electron probe having such high current density impinges thereon.
By the way, the nano-technology extends across information, medical, environmental fields, and, for example, in a micromachine referred to in the medical field, the component constituting the machine becomes less than 1 μm and ready to enter nano order. In addition, the current semiconductor technology is ever being directed to miniaturization, and non-destructive inspection in the class of the resolving power equal to or less than 0.1 μm never before possible using the micro X-ray source becomes a challenge that is required by all means. Especially, in the information field, there is the great challenge of making the line width in the next generation very large scale integrated circuit from 180–130 nm at present to 70–100 nm. Simultaneously, it is often the case where the microstructure consisted principally of a light element become an object to be observed, and, for providing contrast to the image, it becomes an important challenge that the high resolution power is held even in the case of using an X-ray having a long wavelength by the low accelerating voltage of 10 to 20 kV, which has been difficult in the conventional X-ray inspection apparatus.
The invention is achieved in the light of the above described circumstances, and an object of the invention is to provide an X-ray microscopic inspection apparatus for solving the above described various challenges, enabling non-destructive inspection with high resolving power equal to or less than 0.1 μm within a very short period, and capable of largely contributing to the nano-technology field.